Method of absorption-desorption of hydrogen storage alloy and hydrogen storage alloy and fuel cell using said method

ABSTRACT

According to the invention, hydrogen absorbed in a PCT curve low pressure region not desorbed and utilized so far can be desorbed easily by controlling a hydrogen storage alloy temperature in the final stage of a hydrogen desorption process (T2) to a temperature higher than the hydrogen storage alloy temperature in the hydrogen absorption process (T0) and a hydrogen storage alloy temperature in the initial stage of the hydrogen desorption process (T1) (T2&gt;T1≧T0).

TECHNICAL FIELD

[0001] The present invention concerns a hydrogen absorption anddesorption method of repeating pressurization and depressurization ofhydrogen to a hydrogen storage alloy. More specifically, the presentinvention relates to a body-centered cubic hydrogen storage alloy havingtwo-stage plateau characteristics or inclined plateau characteristics.Particularly, the present invention relates to a hydrogen absorption anddesorption method of increasing the amount of desorbed hydrogen within apractical pressure range and temperature range, a hydrogen storage alloysuitable to the absorption and desorption method, as well as a fuel cellusing the hydrogen absorption and desorption method described above.

BACKGROUND ART

[0002] At present, along with increase in the amount of use of fossilfuels such as petroleum, there are fears of acid rains caused by NOx(nitrogen oxides) and also global warming due to increasing CO₂. Sincesuch environmental disruption gives severe problems, intense attentionhas been attracted to the development and practical use of various cleanenergies gentle to the earth. One of new energy development is apractical use of hydrogen energy. Hydrogen is a constituent element ofwater present infinitely on the earth which can be formed by usingvarious primary energies, as well as it can be used as a fluid energyinstead of existent petroleum with no worry of environmental disruptionsince combustion products of hydrogen consists only of water. Further,different from electric power, it has excellent characteristics such asrelatively easy storage thereof

[0003] Accordingly, studies on hydrogen storage alloys as storage andtransportation media of hydrogen have been made vigorously in recentyears, and practical use therefor has been expected. The hydrogenstorage alloys are metals and alloys capable of absorbing and desorbinghydrogen under appropriate conditions. By the use of the alloys,hydrogen can be stored at a lower pressure and at a higher densitycompared with existent hydrogen reservoirs and the volumic densitythereof is substantially equal with or higher than liquid hydrogen orsolid hydrogen.

[0004] For the hydrogen storage alloys, body-centered cubic structuressuch as V, Nb, Ta, or CrTiMn system and CrTiV system (hereinafterreferred to as “BCC”) have been mainly studied as proposed, for example,in Japanese Patent Laid-Open Hei 10-110225. It has been known that suchalloys absorb more hydrogen compared with AB5 type alloys such as LaNi₅or AB2 type alloys such as TiMn₂ put to practical use at present. Thisis because the BCC type alloys have a lot of hydrogen absorbing sitesand H/M is as high as about 2 (H: absorbed hydrogen atoms, M: alloyconstituent elements. About 4.0 wt % in V having an atom weight of about50).

[0005] It has been known that the BCC type alloys having relativelylarge hydrogen absorbing amount take place two-step reactions in thecourse of hydrogen absorption to form hydrides as shown by Reilly and R.H. Wiswall, in Inorg. Chem. 9 (1970), 1678. For example, V reacts withhydrogen at an ambient temperature and forms two kinds of hydridesdepending on the pressure of hydrogen. At first, in the initial stage ofthe reaction where the hydrogen pressure is at a low pressure, anextremely stable hydride is formed as V→VH_(0.8) (α phase→β phase)(hereinafter referred to as “low pressure plateau area”), and thereverse reaction is scarcely taken plate near the room temperature. Whenthe hydrogen pressure is further applied, it forms a hydride as:VH_(0.8)→VH_(2.01) (β phase→γ phase: referred to as high pressureplateau area). Since the equilibrium pressure of hydrogen in thisreaction is at an appropriate pressure of about several atm near theroom temperature, the V-containing BCC alloys have been studiedvigorously as hydrogen storage alloys of high capacity.

[0006]FIG. 1 shows vacuum PCT curves at 313K of a Ti—Cr-xV (Ti/Cr=⅔,X=20-100) body-centered cubic alloy. The low pressure plateau observedat a hydrogen pressure of 0.1 Pa to 10 Pa in FIG. 1 tends to appear onthe side of higher pressure as the addition amount of V is smaller. FIG.2 shows high pressure PCT curves for the same specimen. A flat area near10⁶ Pa in FIG. 2 is a high pressure plateau area and the high pressureplateau area shifts to lower pressure side as the addition amount of Vis smaller. Further, from FIG. 1 and FIG. 2, it is also confirmed thatTi—Cr—V system alloys show two-stage plateaus. FIG. 3 is a PCT curve fora Ti₄₀Cr₅₈Mo₂ alloy which is plotted for the pressure range from 1 Pa to10 MPa, and an inclined plateau is observed in a low pressure region.FIG. 4 is an XRD chart for the alloy and it has been confirmed that aBCC mono-phase is formed by quenching from 1420° C. in iced water. Theinclined area between the inclined plateau area of the low pressureregion and the high pressure plateau areas is a region in accordancewith the Sievert's law. In addition to V, Nb is also a metal havingtwo-stage plateaus (low pressure phase NbH and high pressure phaseNbH2). Further, Ti shows two-stage plateaus transforming as α→β→γ, thisbeing a high temperature operation. Further, FeTi is an intermetalliccompound having two-stage plateaus that operates near 40° C. Further,alloys such as (Zr, Ti)V2 alloys show inclined plateaus and such alloysare also used as hydrogen storage alloys. Further, the hydrogen storagealloys showing the two-stage plateau or the inclined plateau have afeature that the PCT characteristic curve is in contact with three ormore parallel lines, or a feature of having three or more knick pointsin the PCT curve within the pressure range from a low pressure of 1 Paor less to 10 MPa. For example, in the Ti₄₀Cr₅₈Mo₂ alloy shown in FIG.3, it can be easily confirmed that the PCT curve is in contact with fourparallel lines and also has four knick points each shown by an arrow. Onthe contrary, the PCT characteristic curves of existent AB5 type alloyssuch as an LaNi₅ alloy are in contact with two parallel lines and havetwo knick points.

[0007] The prior art which is considered to be based on the idea forattaining a hydrogen storage alloy of high capacity by the two-stageplateau and inclined plateau characteristics described above includesthe followings; (a) a spinodal curve decomposition structure isdeveloped in a Ti alloy of a body-centered cubic structure (JP-A No.10-110225), (b) Cu and/or rare earth element is added to Ti—Cr—V systemalloys (JP-B No. 4-77061), (c) a molten Ti alloy is quenched to form aBCC mono-phase at a room temperature (JP-A No. 10-158755), and (d) alattice constant of a BCC alloy comprising Ti—Cr as a main element iscontrolled (JP-A No. 7-252560).

[0008] Among the hydrogen absorption and desorption methods describedabove, JP-A No. 10-110225 and JP-A No. 7-252560 include descriptionsregarding hydrogen absorbing and desorbing temperature. Each of themethods conducts hydrogen absorption and desorption at a constanttemperature. In the latter JP-A No. 7-252560, the activatingpre-treatment is conducted in two steps, that is, a pre-stage at lowtemperature and post stage but the absorbing and desorbing temperatureis constant (20° C.). A method of absorbing hydrogen to a hexagonalsystem Ti—Cr—V system alloy which is not a BCC alloy and heating at 100°C. (column 4, lines 32-39) in JP-B No. 59-38293 is also an absorptionand desorption method at a constant temperature.

[0009] On the other hand, application use utilizing hydrogen describedabove includes fuel cells. Since the fuel cells have higher powergeneration efficiency compared with thermal power generation, they havebeen studied vigorously and remarkable improvement for the powergeneration efficiency in the feature has been expected. As the fuel forthe fuel cell, hydrogen in natural gas or methanol, etc. is utilized.Since the fuel cells using hydrogen as the fuel are simple in thestructure and exhibit excellent performance, alkali electrolyte type andsolid polymeric film type fuel cells with a power of about 10 kW havebeen used as energy sources for mobile engines such as satellites,deep-sea vessels, and electric automobiles. Further, application in ageneral range is expected, for example, as portable fuel cells and powersources for use in portable equipments.

[0010] In the hydrogen storage alloy having the two-stage plateaucharacteristics such as the V-containing BCC alloys described abovewhich have been generally studied as hydrogen storage alloys of highcapacity, since the hydrogen absorbing reaction in a low pressureplateau area proceeds only to the side reacting with hydrogen at a roomtemperature, it was not practiced so far to take out hydrogen absorbedin the low pressure plateau area and use it as effective hydrogen.Generally, it is said that the amount of hydrogen taken out ofbody-centered cubic hydrogen storage alloys including pure V and pure Nbis extremely small relative to the theoretical amount (New edition,Hydrogen Storage Alloy—Physical Property and Application UseThereof—written by Yasuaki Osumi, published from Agne Techno Center (newedition, first print, Feb. 5, 1992), pages 340-341).

[0011] In AB5 type alloys such as LaNi₅ or BCC type alloys put topractical use at present, the equilibrium pressure with hydrogen can becontrolled by controlling the alloy ingredients. Further, while theequilibrium pressure of hydrogen storage alloy with hydrogen can becontrolled by the operation temperature, the existent development foralloys lacks in the technical idea of effectively utilizing the hydrogenabsorbing characteristics in the low pressure area of the PCTcharacteristic curve. That is, for increasing the capacity of absorbedhydrogen in the BCC type hydrogen storage alloys, while it is consideredeffective that hydrogen in the reaction between α phase→β phase, thatis, in the low pressure area of the PCT characteristics curve (reactionof V→VH_(0.8), for example, for V), in addition to the β phase region(region between the low pressure plateau area and the high pressureplateau area in accordance with the Sievert's law) of BCC type alloys isconcerned with the absorption and desorption reaction, but the methodhas not yet been disclosed.

[0012] Accordingly, the present invention intends to provide a hydrogenabsorption and desorption method capable of absorbing and desorbing morehydrogen by effectively utilizing not only the reaction between αphase→β phase but also hydrogen therebetween, that is, in the lowpressure region of the PCT curve, for pure V and pure Nb or solidsolubilized materials shown hydrogen absorbing/desorbing reactivitiessimilar to those of the pure V and Nb materials, as well as solidsolubilized BCC alloys such as Ti—Cr based alloys exhibiting thetwo-stage plateau characteristics or inclined plateau characteristics,alloys suitable to the method described above, as well as a fuel cellusing the method and the method of using the same.

DISCLOSURE OF THE INVENTION

[0013] The hydrogen absorption and desorption method according to thepresent invention for solving the subject described above is a method ofabsorbing and desorbing hydrogen by repeating pressurization anddepressurization of hydrogen properly for a body-centered cubic hydrogenstorage alloy exhibiting two-stage plateau characteristics or inclinedplateau characteristics in which a hydrogen storage alloy temperature inthe final stage of a hydrogen desorption process (T2) is controlled to atemperature higher than a hydrogen storage alloy temperature in thehydrogen absorption process (T0) and a hydrogen storage alloytemperature in the initial stage of the hydrogen desorption process (T1)(T2>T1≧T0). The two-stage plateau characteristic or the inclined plateaucharacteristics specifically mean that a PCT curve showing equilibriumcharacteristics of reaction between the hydrogen storage alloy andhydrogen is in contact with three or more parallel lines, or the PCTcurve has three or more knick points. The measuring range for the PCTcurve referred to herein is a range from a low pressure of 0.1 Pa orless to a high pressure of about 10 MPa.

[0014] Further, the hydrogen storage alloy of the invention hastwo-stage plateau characteristics or inclined plateau characteristics inthe PCT curve wherein hydrogen utilizable effectively can be increasedby making the low pressure region of the PCT curve instable.

[0015] According to the features described above, hydrogen absorbed inthe low pressure region of the PCT curve that was neither desorbed norutilized in the prior art can be desorbed easily and can be taken out asutilizable hydrogen by making the hydrogen storage alloy temperature inthe final stage of the desorption process (T2) to a high temperatureand, as a result, the amount of hydrogen utilizable in the hydrogenstorage alloy can be increased.

[0016] In the Ti—Cr—V alloy of FIG. 1, the plateau stability in the lowpressure region varies depending on the addition amount of V. That is,in an alloy having a low pressure plateau or an inclined plateau in alow pressure region of a PCT curve, hydrogen present in the low pressureregion of the PCT curve can be made into an effectively utilizable formby changing the composition. A portion of hydrogen in the low pressureregion of the PCT curve which is rendered instable can be utilizedeffectively by making the temperature in the desorption process (T2)higher than the temperature in the hydrogen absorption process (T0) inthis state. FIG. 5 shows vacuum PCT curves of specimens obtained bykeeping a Ti₂₄Cr₃₆V₄₀ alloy at 1673K for one hour and then rapidlyquenching the same in iced water. When comparing a vacuum PCT curvemeasured at an environmental temperature of 368K with a vacuum PCT curvemeasured at 313K, it can be seen that the residual amount of hydrogen inthe hydrogen storage alloy can be decreased for an identical hydrogenpressure as the dehydrogenation temperature is higher. At 0.01 MPaattainable by evacuation of a rotary pump, the amount of hydrogen can bedecreased by 0.12% by weight by dehydrogenation at 368K compared withthe case of dehydrogenation at 313K At a further lower hydrogenpressure, since the effect of instabilizing the plateau becomesremarkable and the amount of hydrogen can be decreased more, morehydrogen can be absorbed in the hydrogen absorption process. Incomparison, in a case of an LaNi₅ based alloy having no low pressureplateau, increment of hydrogen by temperature elevation is at most about0.05 wt % at 0.01 MPa and, at a further lower hydrogen pressure, theamount of effectively utilizable hydrogen is decreased further (FIG. 6).

[0017] The hydrogen storage alloy temperature in the initial stage ofthe desorption process (T1) is made lower than the hydrogen storagealloy temperature in the final stage of the desorption process (T2)(T2>T1), in order to suppress the cycle deterioration on the effectivehydrogen absorbing capacity of the alloy. T1 is sometimes controlled tohigher than the hydrogen storage alloy temperature in the absorptionprocess (T0) in order to control the hydrogen desorption rate of thealloy, and the amount of utilizable hydrogen can also be increased inthis case by setting as: T2>T1 in the final desorption process.Temperature elevation conducted only in the initial stage of hydrogendesorption or only temporarily has an effect of increasing the hydrogendesorbing rate but the amount of effectively utilizable hydrogen is notincreased. For increasing the amount of effectively utilizable hydrogen,it is effective to elevate the temperature in the final stage of thehydrogen desorption process. The final stage of the hydrogen desorptionprocess is at or after any instance where the residual amount ofhydrogen in the hydrogen storage alloy is reduced to 50% or less, morepreferably, 25% or less, and temperature elevation in the final stage ofthe process is effective for the suppression of the cycle deterioration.

[0018] Since it is more practical as the hydrogen absorbing alloytemperature in the hydrogen desorption process T1 is nearer to the roomtemperature, it is preferred that T1≦373K.

[0019] The hydrogen storage alloys suitable to applications of thehydrogen absorption and desorption method of the invention and capableof obtaining a large amount of effective hydrogen absorption capacityare body-centered cubic alloys represented by the general formula:Ti_(X)Cr_(Y)M_(Z) in which M is one or more members selected fromelements belonging to the groups IIa, IIIa, IVa, Va, VIa, VIIa, VIII,IIIb, IVb of the periodical table, in 20≦X+Y≦100 atomic %, 0.5≦Y/X≦2 and0<≦80 atomic %, and including inevitably intruded oxygen or nitrogen andinevitably forming minimum spinodal decomposition phase. Addition of oneor more members selected from the elements belonging to the groups IIa,IIIa, IVa, Va, VIa, VIIa, VIII, IIIb, IVb of the periodical table to aTi—Cr binary alloy gives an effect of not only stabilizing thebody-centered cubic structure but also of instabilizing the PCT curvelow pressure area. In this alloy, the Cr/Ti ratio is defined as0.5≦Y/X≦2, because the plateau pressure is greatly deviated from anormal pressure if the rate is out of the range described above, whichis not practical. Particularly, since oxygen deteriorates the effectiveabsorption amount of hydrogen, it is preferably as less as possible.Further, since the effective absorption amount of hydrogen is decreasedwhen the spinodal decomposition phase is formed, lowering of theabsorption amount can be suppressed by not applying a heat treatmentthat tends to cause spinodal decomposition or shortening the treatingtime.

[0020] When the alloy is set as a body-centered cubic hydrogen storagealloy comprising V at 60 atomic % or less, and/or Mo, Al, Mn and/or rareearth elements at 10 atomic % or less for the constituent M, 2.5% byweight or more effective absorption amounts of hydrogen can be obtainedin method determined original point on evaccuation, and the hydrogenabsorption and desorption method of the invention can be utilized moreefficiently.

[0021] On the other hand, according to the measurement by methoddetermined original point on evaccuation, the effective absorptionamount of hydrogen in the prior art alloys remains at about 2% byweight. While BCC mono-phase can be formed within a compositional Vrange from 5 to 100 atomic %, since the stability of VH_(0.8) formed asa hydride product of pure V is remarkably lowered by lowering the amountof admixed V to 60% or less, effective utilization of hydrogen in thePCT curve low pressure region is facilitated. Further, since V is anexpensive element as well, an excess amount of admixed V over 60 atomic% will lead to difficulty in practical use.

[0022] Among the additive elements described above, Mo, Al, and Mn servestabilizers for the BCC phase by the addition of a small amount and havean effect of suppressing the formation of a Laves phase thatdeteriorates the absorption amount of hydrogen, thereby increasing theeffective absorption amount of hydrogen. When the addition amount of Mo,Al, Mn, and rare earth elements exceeds 10%, the hydrogen absorptionamount is decreased remarkably, so that the elements are preferablycontained at 10% or less. FIG. 7 shows high pressure PCT curves ofTi₂₇Cr_(43-X)Mn_(X)V₃₀ (in which X=10, 15, 20 atomic %) BCC alloymeasured at 40° C. by method determined original point on evaccuation.As shown in the graph, when the Mn addition amount is 10 atomic %, theeffective hydrogen absorption amount shows a satisfactory value as 2.6%by weight but as the Mn addition amount is increased to 15% and 20%, theeffective absorption amount of hydrogen lowers remarkably to a valueless than 2% by weight. The effective absorption amount of hydrogen ofthe alloy in which the addition amount of V is changed to 20 atomic %and controlled to show a plateau near the normal pressure issubstantially equal with a case where the addition amount of V is 30atomic % and it is considered that the effective absorption amount ofhydrogen depends on the addition amount of Mn. The trend is identicalalso for Mo, Al and rare earth elements. In a case of utilizing startingmaterials at a low purity, since the rare earth elements act as a getterfor oxygen or the like intruded as impurities, addition of the rareearth elements by a small amount is also effective for suppressingdeterioration by oxygen and maintaining high characteristics.

[0023] On the other hand, the fuel cell according to the presentinvention has a feature comprising a hydrogen storage tank incorporatinga hydrogen storage alloy having two-stage plateau characteristics orinclined plateau characteristics, a temperature control device forelevating or cooling the temperature of the hydrogen storage alloydirectly or the atmospheric temperature of the absorption alloy, a fuelcell capable of outputting electric power via chemical change ofhydrogen supplied from the hydrogen storage tank and a control sectionfor controlling such that a hydrogen absorption alloy temperature in thefinal stage of hydrogen desorption process (T2) is made to a temperaturehigher than a hydrogen storage alloy temperature in a hydrogenabsorption process (T0) and a hydrogen storage alloy temperature in theinitial stage of the hydrogen desorption process (T1) (T2>T1≧T0).According to the feature, since the temperature of the hydrogen storagealloy in the final stage of the hydrogen desorption process (T2) is madehigher than the temperature in the hydrogen absorption process (T0),hydrogen absorbed in the PCT curve low pressure region which was neitherdesorbed from the hydrogen storage alloy nor utilized in the prior artcan be taken out as a utilizable hydrogen and the electric powerobtained from the fuel cell can be increased. Further, since thetemperature T2 is made higher than the hydrogen storage alloytemperature in the initial stage of the hydrogen desorption process (T1)the life of the fuel cell can be increased.

[0024] In the fuel cell according to the invention, it is preferred thatthe control section can properly control the pressure, temperature andflow rate of the hydrogen gas supplied to the hydrogen storage tank andthe fuel cell. With the constitution described above, by controlling thepressure, the temperature and the flow rate of the hydrogen gas, theamount of electric power generation in the fuel cell can be properlycontrolled depending on the load and the utilization efficiency ofhydrogen used in the fuel cell can be improved.

[0025] In the fuel cell according to the invention, it is preferred thatthe temperature control device described above can utilize the heatdissipated from the fuel cell or the heat of exhaust gases exhaustedfrom the fuel cell for the temperature elevation. With the constitutiondescribed above, since the dissipated heat or discharged heat from thefuel cell can be utilized for the temperature elevation of the hydrogenstorage alloy, electric power, etc. are no more required for thetemperature elevation of the hydrogen storage alloy, which can improvethe efficiency in the overall hydrogen fuel cell.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026]FIG. 1 is vacuum PCT curves for a Ti—Cr—V_(X) (Ti/Cr=⅔, X=20-100)alloy.

[0027]FIG. 2 is high pressure PCT curves for a Ti—Cr—V_(X) (Ti/Cr=⅔,X=20-100) alloy.

[0028]FIG. 3 is a PCT curve of a Ti₄₀Cr₅₈Mo₂ alloy and four parallellines and knick points.

[0029]FIG. 4 is an XRD chart for a Ti₄₀Cr₅₈Mo₂ alloy.

[0030]FIG. 5 is vacuum PCT curves for a specimen of a Ti₂₄Cr₃₆V₄₀ alloyafter keeping at 1673K for 1 hour and quenching in iced water.

[0031]FIG. 6 is PCT curves for an LaNi₅ alloy.

[0032]FIG. 7 is high pressure PCT curves for a Ti₂₇Cr_(43-X)Mn_(X)V₃₀(X=10, 15, 20 atomic %) BCC alloy measured by method determined originalpoint on evaccuation.

[0033]FIG. 8 is an XRD charts for a Ti₃₉Cr_(57.5)Mo_(2.5)La₁ alloy, aTi_(39.5)Cr₅₆Mo_(2.5)Al₁La₁ alloy and a Ti_(35.5)Cr_(50.5)Mo₂Mn₅V₇alloy.

[0034]FIG. 9 is vacuum PCT graphs for a Ti₃₉Cr_(57.5)Mo_(2.5)La₁ alloy.

[0035]FIG. 10 is high pressure PCT graphs for aTi_(39.5)Cr_(57.5)Mo_(2.5)La₁ alloy, a Ti_(39.5)Cr₅₆Mo_(2.5)Al₁La₁ alloyand a Ti_(35.5)Cr_(50.5)Mo₂Mn₅V₇ alloy.

[0036]FIG. 11 shows dependence of the hydrogen absorption amount of aTi₃₉Cr_(57.5)Mo_(2.5)La₁ alloy, a Ti_(39.5)Cr₅₆Mo_(2.5)Al₁La₁ alloy anda Ti_(35.5)Cr_(50.5)Mo₂Mn₅V₇ alloy on the number of cycle tests.

[0037]FIG. 12 is a system flow chart showing an embodiment of a fuelcell according to the invention.

[0038]FIG. 13 is a model view showing a mechanism of forming electricpower in a fuel cell used for the fuel cell according to the invention.

BEST MODE FOR PRACTICING THE INVENTION

[0039] Embodiments of the present invention are to be described withreference to the drawings.

EXAMPLE 1

[0040] This example shows possibility for excellent absorption amountand effective suppression of cycle deterioration, by using abody-centered cubic hydrogen storage alloy having an inclined plateau ina PCT curve low pressure region and controlling a hydrogen storage alloytemperature in the final stage of a hydrogen desorption process (T2) toa temperature higher than a hydrogen storage alloy temperature in thehydrogen absorption process (T0) and a hydrogen storage alloytemperature in the initial stage of the hydrogen desorption process (T1)(T2>T1≧T0).

[0041] After weighing commercially available starting materials, themixtures were subjected to arc melting in argon with a water cooledcopper hearth to prepare a Ti₃₉Cr_(57.5)Mo_(2.5)La₁ alloy, aTi_(39.5)Cr₅₆Mo_(2.5)Al₁La₁ alloy and a Ti_(35.5)Cr_(50.5)Mo₂Mn₅V₇ alloyeach by 25 g. After coarsely pulverizing the alloys with a stamp mill,they were kept at 1723K for 10 min and then quenched in iced water. Thephase appearing in the quenched alloy specimens were identified by anX-ray diffraction apparatus manufactured by Shimazu, Japan. Highpressure PCT characteristics and vacuum PCT characteristics weremeasured by using PCT characteristic measuring apparatus manufactured bySuzuki Shokan. A cycle test was conducted also by the PCT characteristicmeasuring apparatus.

[0042]FIG. 8 shows XRD charts for specimens after quenched in icedwater. All the prepared specimens had BCC mono-phase. FIG. 9 showsvacuum PCT characteristics of a Ti₃₉Cr_(57.5)Mo_(2.5)La₁ alloy. When thetemperature was elevated from 40° C. to 100° C., instabilization of theinclined plateau in the low pressure region was confirmed. FIG. 10 showshigh pressure PCT characteristics before the cycle test of theTi_(39.5)Cr_(57.5)Mo_(2.5)La₁ alloy, a Ti_(39.5)Cr₅₆Mo_(2.5)Al₁La₁ alloyand a Ti_(35.5)Cr_(50.5)Mo₂Mn₅V₇ alloy. Presence of the phase stabilizedat low pressure is confirmed by FIG. 9. In view of the drawings, it canbe easily confirmed that the PCT curve is in contact with three or moreparallel lines and has three or more knick points. Table 1 shows thedependence on the dehydrogenation temperature of the hydrogen absorptionamount by method determined original point on evaccuation of theTi₃₉Cr_(57.5)Mo_(2.5)La₁ alloy, a Ti_(39.5)Cr₅₆Mo_(2.5)Al₁La₁ alloy anda Ti_(35.5)Cr_(50.5)Mo₂Mn₅V₇ alloy. TABLE 1 Hydrogen absorption amount(method determined original point on evaccuation) Alloy composition 40°C. 100° C. Invention Ti₃₉Cr_(57.5)Mo_(2.5)La₁ 2.68 2.88Ti_(35.5)Cr_(50.5)Mo₂Mn₅V₇ 2.72 2.91 Ti₃₉Cr_(57.5)Mo_(2.5)Al₁La₁ 2.612.83 Comparative LaNi_(4.5)Al_(0.5) 1.41 1.42 Example

[0043] As described above, BCC type hydrogen storage alloys show largeeffective absorption amount of hydrogen and the absorption amount can beincreased further by desorption at higher temperature. Compared withcomparative example LaNi₅, it can be seen that the effect of utilizingthe temperature difference of the alloy according to the invention isremarkably large.

[0044] Then, a cycle test was conducted on the alloys by using a highpressure PCT characteristic evaluation apparatus. The hydrogenabsorption process was set at 40° C. in all of the cases. In thecomparative example, desorption was conducted at 100° C. throughout theprocess of desorption and only the final stage of the desorption processwas set at 100° C. in the invention. FIG. 11 is a graph showing relationbetween the number of the cycle test and the absorption amount ofhydrogen for a Ti₃₉Cr_(57.5)Mo_(2.5)La₁ alloy, aTi_(39.5)Cr₅₆Mo_(2.5)Al₁La₁ alloy and a Ti_(35.5)Cr_(50.5)Mo₂Mn₅V₇alloy. It can be seen that the result shown by solid lines in which thetemperature was elevated to 100° C. only in the final stage of thedesorption process shows more excellent cycle characteristics over thatby broken lines where temperature was set to 100° C. throughout theperiod of desorption Further, Table 2 shows relation between thedesorption temperature control and cycle deterioration for aTi₃₉Cr_(57.5)Mo_(2.5)La₁ alloy, a Ti_(39.5)Cr₅₆Mo_(2.5)Al₁La₁ alloy anda Ti_(35.5)Cr_(50.5)Mo₂Mn₅V₇ alloy. TABLE 2 Hydrogen absorption amountafter 100 cycles 100° C. throughout 100° C. in the final stage Alloycomposition desorption of desorption (atomic %) (Comp. Example)(Invention) Ti₃₉Cr_(57.5)Mo_(2.5)La₁ 81.5% 92.2%Ti_(35.5)Cr_(50.5)Mo₂Mn₅V₇ 80.7% 89.0% Ti₃₉Cr_(57.5)Mo_(2.5)Al₁La₁ 82.4%91.8%

[0045] As described above, cycle deterioration can be suppressed bycontrolling the temperature in the final stage of the desorption processis higher than that in the initial stage of the desorption process.Accordingly, it can be seen that large effective absorption amount ofhydrogen can be attained and cycle deterioration can be suppressed byeffectively utilizing hydrogen in the PCT curve low pressure regionusing the invention.

EXAMPLE 2

[0046] This example shows a constitutional view of a fuel cell having afeature comprising hydrogen storage tank incorporating a hydrogenstorage alloy, a temperature control device for elevating or cooling thetemperature of the hydrogen storage alloy directly or the atmospherictemperature of the absorption alloy, a fuel cell capable of outputtingelectric power by chemical change of hydrogen supplied from the hydrogenstorage tank and a control section for controlling such that a hydrogenabsorption alloy temperature in the final stage of hydrogen desorptionprocess (T2) is made to a temperature higher than a hydrogen storagealloy temperature in a hydrogen absorption process (T0) and a hydrogenstorage alloy temperature in the initial stage of the hydrogendesorption process (T1) (T2>T1≧T0), and a method of increasing theamount of electric power obtained by the fuel cell and suppressing cycledeterioration.

[0047]FIG. 12 shows a system flow chart showing an embodiment of a fuelcell according to the present invention. A hydrogen fuel tank 4 is atank for supplying hydrogen to a fuel cell to be described later and thetank is incorporated with a body-centered cubic hydrogen storage alloyhaving two-stage plateau characteristics or inclined plateaucharacteristics. The tank is provided with a solenoid valve V11 forintroducing starting hydrogen, as well as a solenoid valve V1 forsupplying hydrogen to the fuel cell and a solenoid valve V2 forrecovering the hydrogen returned from the fuel cell to the tank disposedbetween the tank and the fuel cell 1, and they are adapted to supplyhydrogen by a pump P2. Further, pressure valves B1 and B2 and flowmeters FM are provided in the course of the pipeline for controlling thepressure and the flow rate of hydrogen, and the entire system iscontrolled, including temperature by the control device 3. A heatexchanger 5 controlled by the control device is utilized for temperatureelevation and temperature lowering of a hydrogen storage alloy. In theheat exchanger 5, heat exchange is conducted between exhausted heatpossessed in steams at a relatively high temperature exhausted from thefuel cell 1 and cold water as a cold temperature medium and temperaturesensors TS1-TS3 or the flow meters FM and the pumps are controlled tocontrol the temperature to an aimed level. From the fuel cell 1, a DCpower can be obtained by reaction between oxygen and hydrogen and aninverter 2 for converting the DC power into a predetermined AC power isconnected with the fuel cell. In an application use for supplyingelectric power to electronic equipments, a DC/DC converter may beconnected instead of the inverter 2. LS in the drawing is a water levelsensor in an accumulation tank for accumulating water formed when steamsexhausted from the fuel cell are cooled by the heat exchanger 5.

[0048] Then, the operation of the fuel cell of the invention is to bedescribed. At first, when a hydrogen reservoir at a high pressure isconnected with a hydrogen supply port of the tank 4 and the solenoidvalve V11 is opened, the hydrogen fuel supplied into the tank andhydrogen is absorbed from the low pressure region up to the highpressure region shown by the PCT curve for the hydrogen storage alloyincorporated in the tank. In this case, the pump 5 is operated to sendexternal air into the heat exchanger and a circulation pump 3 iscontrolled properly such that the temperature of the tank (T0) is 40° C.or lower. When the absorption is completed, the solenoid valve V11 isclosed.

[0049] When the fuel cell is operated, signals from various kinds ofsensors are received by the control device 3, opening/closure of thesolenoid valves V1 and V2 and the pressure valves B1 and B2 arecontrolled to supply hydrogen to the fuel cell 1. In this step, controlfor supplying heat from the heat exchanger 5 is also conducted forcontrolling the rate of supplying hydrogen. In the invention, the alloytemperature in the final stage of hydrogen desorption process (T2) iscontrolled to higher than the alloy temperature in the initial stage ofthe hydrogen desorption process (T1), to effectively utilize hydrogenabsorbed in the hydrogen storage alloy, particularly, hydrogen in thePCT curve low pressure region.

[0050] Hydrogen is thus supplied to the fuel cell 1 and, at the sametime, oxygen is supplied from the oxygen electrode in which oxygen andhydrogen are reacted to obtain electric power in the fuel cell. As shownin FIG. 13, the reaction provide electric power by using a reactionopposite to that formed by hydrogen and oxygen through electrolysis ofwater when a DC current is applied to a water incorporated with anelectrolyte. Hydrogen molecules supplied from the hydrogen fuel tank 4release electrons at the hydrogen electrode to form hydrogen ions andthe electrons are transferred toward the anode to obtain electric power.

[0051] The hydrogen ions transfer through the electrolyte toward theanode, accept electrons at the anode and return to hydrogen atoms, whichreact with oxygen to form water (steams), and exhaust gases containingsteams at a relatively high temperature (about 70 to 90° C.) are formedby the heat of reaction. By controlling the exhaust gases to flow intothe heat exchange by way of the valve, they can utilized as a heatsource for heating.

[0052] Upon starting of power generation, since hydrogen supplied fromthe hydrogen reservoir is hydrogen in the high pressure plateau regionof the hydrogen storage alloy, and it is easily released, it can beutilized being controlled to a temperature near the hydrogen absorptiontemperature (T0). When the desorption of hydrogen continues and thehydrogen desorption from the high pressure plateau region of thehydrogen absorption alloy decreases, cold water elevated for thetemperature by way of the heat exchanger 5 is supplied to the tank tostart heating for the hydrogen storage alloy. Hydrogen absorbed in thePCT curve low pressure region can now be utilized effectively by theheating to greatly improve the power generation capacity of the fuelcell.

[0053] Electric power obtained could be increased by about 14% comparedwith the case of desorption at a temperature constant at 20° C. by usingthe Ti₃₉Cr₅₇Mo₃La₁ alloy for the hydrogen absorption tank, absorbinghydrogen at 20° C. and desorbing the same at 85° C. in the final stageof desorption process. Further, the life of the tank at which theabsorption amount of hydrogen was reduced to 90% of the initial valuewas extended by about 30% compared with a case of keeping the hydrogenstorage alloy temperature in the desorption process (T2) constant at 85°C. from the initial stage.

[0054] In this example, the upper limit for T2 was defined at 90° C.since water was used as cold temperature medium but the invention is notlimited only thereto and heating by a heater can also be utilized. Inthe same manner, it is also possible to utilize a coolant other thanwater for cooling or utilize the method of enabling both cooling andheating, for example, by a Peltier device.

1. A hydrogen absorption and desorption method for a hydrogen storagealloy which comprises steps of absorbing and desorbing hydrogen byproperly repeating hydrogen pressurization and depressurization to saidhydrogen storage alloy being a body-centered cubic hydrogen storagealloy having two-stage plateau characteristics or inclined plateaucharacteristics wherein a hydrogen storage alloy temperature in thefinal stage of hydrogen desorption process (T2) is controlled to atemperature higher than a hydrogen storage alloy temperature in ahydrogen absorption process (T0) and a hydrogen storage alloytemperature in the initial stage of the hydrogen desorption process (T1)(T2>T1≧T0).
 2. The hydrogen absorption and desorption method accordingto claim 1, wherein the hydrogen storage alloy temperature in the finalstage of the hydrogen desorption process (T2) is 150° C. or lower. 3.The hydrogen absorption and desorption method according to claim 1 or 2,wherein the process at or after the instance where hydrogen in thehydrogen storage alloy is decreased to any residual amount of 50% orless in the hydrogen desorption process is defined as the final stagefor the hydrogen desorption process.
 4. The hydrogen absorption anddesorption method according to any of claims 1 to 3, wherein the processat or after the instance where hydrogen in the hydrogen storage alloy isdecreased to any residual amount of 25% or less in the hydrogendesorption process is defined as the final stage for the hydrogendesorption process.
 5. A body-centered cubic hydrogen storage alloy forconducting absorption and desorption of hydrogen in the reaction betweenthe hydrogen storage alloy and hydrogen in which a hydrogen storagealloy temperature in the final stage of a hydrogen desorption process(T2) is controlled to a temperature higher than a hydrogen storage alloytemperature in the hydrogen absorption process (T0) and higher than ahydrogen storage alloy temperature in the initial stage of hydrogendesorption (T1) (T2>T1≧T0) wherein the hydrogen storage alloy hastwo-stage plateau characteristics or inclined plateau characteristics.6. The hydrogen storage alloy according to claim 5 wherein said hydrogenstorage alloy has a composition represented by the general formula:Ti_(X)Cr_(Y)M_(Z) in which M is one or more members selected fromelements belonging to the groups IIa, IIIa, IVa, Va, VIa, VIIa, VIII,IIIb, and IVb of the periodical table, 20≦X+Y<100 atomic %, 0.5≦Y/X≦2,0<Z≦80 atomic %, and includes inevitably intruded oxygen or nitrogen andminimum spinodal decomposition phase formed inevitably.
 7. The hydrogenstorage alloy according to claim 6, wherein the additive element M is Vat 60 atomic % or less.
 8. The hydrogen storage alloy according to claim6 or 7, wherein the additive element M is one or more members selectedfrom Mo, Al, Mn and rare earth elements at 10 atomic % or less.
 9. Afuel cell comprising a hydrogen storage tank incorporating a hydrogenstorage alloy, a temperature control device for elevating or cooling atemperature directly of the hydrogen storage alloy or an atmospherictemperature of the absorption alloy, a fuel cell capable of outputtingelectric power by chemical change of hydrogen supplied from the hydrogenstorage tank, and a control section for conducting control that ahydrogen storage alloy temperature in the final stage of a hydrogendesorption process (T2) is at a temperature higher than a hydrogenstorage alloy temperature in a hydrogen absorption process (T0) and ahydrogen storage alloy temperature in the initial stage of the hydrogendesorption process (T1) (T2>T1≧T0).
 10. The fuel cell according to claim9, wherein the control section is adapted to properly control thepressure, temperature and flow rate of the hydrogen gas supplied to thehydrogen storage tank and the fuel cell.
 11. The fuel cell according toclaim 9 or 10, wherein the temperature control device can utilize a heatdissipated from the fuel cell or a heat of exhaust gases exhausted fromthe fuel cell for the temperature elevation.